Phase Shift Keyed (PSK) modulators are known in the art, the resulting modulation of such devices generally modifies the phase of a carrier in accordance with the intelligence desired to be transmitted. A generalized discussion of modems can be found in the paper with that title by J. R. Davey, appearing in the Proceedings of the IEEE, Volume 60, pages 1284-1292 (November 1972). See also "Multi-Level PSK Modems for TDMA Systems" by Noguchi in the 1975 EASCON, and "Design of a PSK Modem for the Telsat TDMA System" by Yokoyama et al in ICC 1975.
One particular form of PSK is known as quartenary phase shift keying (QPSK) in which in phase and quadrature carrier components are separately modulated by different bit streams; the modulated carrier components are then combined for transmission purposes. Typically in using a QPSK modulator a digital bit stream which carries the modulating intelligence is separated into two different bit streams (sometimes called I and Q channels) wherein alternate bits in the original stream appear on separate channels, and the bit rate on each of the I and Q channels is 1/2 the original bit rate. Corresponding bits on the I and Q channels correspond to a symbol, and the transmitted symbol rate is again equal to 1/2 the original bit rate.
FIG. 1 illustrates a typical prior art QPSK modulator. As shown, the modulating signal source 10 (at a bit rate 1/T) provides a digital signal stream to a bit separation and stretching circuit 11 which also receives I and Q clock pulses from a clock pulse source 12. The output of the bit separation and stretching circuit 11 is an I and Q channel, each of which carry digital signal streams whose bit rate is 1/2 the original bit rate of the modulating signal, and in which successive bits from the modulating signal source 10 appear on separate I and Q channels. The modulator itself includes a filter 13 for each of the I and Q channels. The filtered I and Q bit streams are provided to amplitude modulators 14 for each channel. The amplitude modulators 14 also receive I and Q signals from an IF source 15. The IF source 15 corresponds to the carrier and the I and Q signals from source 15 are phase displaced by 90.degree.. The output of the amplitude modulators are then summed in a summing device 16, and the resulting summed signal is provided to a high powered amplifier the output of which is coupled to an antenna for transmission after appropriate frequency upconversion at an appropriate radio frequency.
FIG. 2 illustrates an alternate version of the typical QPSK modulator. FIG. 2 in most respects is similar to FIG. 1 except that the modulator of FIG. 2 has omitted the filters 13, coupling the I and Q signals to the amplitude modulators 14 and instead provides a bandpass filter 17 coupled between the summing device 16 and the high powered amplifier (or HPA).
Efficient modulator design requires an understanding of the particular application so that the modulator characteristics can be most effectively matched with the environment in which it operates.
For example, one class of applications for modulators is in the satellite communications field, and more particularly, in systems employing Time Division Multiple Access (TDMA). In this configuration, the transmitters transmit in burst or discontinuous fashion, and accordingly, the modulators should be effective in this operating mode.
Another particular characteristic of transmitters which can be tailored by tailoring the modulator is reduction in adjacent channel interference which is also of particular importance in the TDMA satellite communication systems or at least those systems with multiple, frequency separated transponders.
FIG. 3 is a block diagram of such a TDMA satellite transmission system environment. FIG. 3 illustrates three representative earth stations; of course, typical systems would employ many more than three earth stations. For each earth station, FIG. 3 illustrates a PSK modulator, a filter F1 and an HPA; of course, many other devices are in the signal chain, but these are particular devices of interest. The filtering represented by filter F1 is the filtering associated with the modulator. The satellite equipment includes an input filter F2, a travelling wavetube amplifier (TWTA) and an output filter F3. The receiving earth station includes further filtering (filter F4) and a PSK demodulator.
For analysis purposes, the transmission path of interest originates from the second earth station. That station's transmissions are affected by noise contributions from a variety of sources. Those sources include adjacent channel interference (ACI), potential up-path rain attenuation (A.sub.u) and up-path thermal noise at the satellite receiver (N.sub.u). The channel of interest is also affected, at the receiving earth station, by down-path thermal noise (N.sub.d) as well as adjacent channel interference (ACI). At the satellite receiver, the modulated carrier, combined with noise and adjacent channel interference, is frequency translated from the up-link frequency (e.g., 14 GHz.) down to the down-link frequency (e.g., 12 GHz.) and filtered. It is then amplified by the satellite TWTA and again filtered before transmission to a receive earth station. In the down-link transmission, additional thermal noise and adjacent channel interference is contributed by the earth station system noise and by downpath interference from neighboring channels.
Earth stations numbers 1 and 3 transmit in respective TDMA bursts through channel transponders whose frequency assignments are adjacent to the channel of interest.
In more detail, the digital bit stream (digitized voice or data) first modulates the carrier in the PSK modulator. Waveform shaping, which can take place at IF or baseband, is performed by the filter F1. The modulated IF is upconverted to the up-link frequency, amplified in the HPA and fed to the transmit antenna. The up-link path may be subjected to rain attenuation (A.sub.u). At the satellite receiver, thermal noise (N.sub.u) is introduced and ACI is contributed by out of band energy of PSK/TDMA carriers accessing transponders occupying adjacent frequency slots. This out of band energy is due to PSK spectrum spreading caused by non-linear characteristics of the HPA. FIG. 4 is an example of a QPSK spectrum after spreading takes place. The level of ACI can be decreased when the HPA operating point is moved toward a more linear region (sometimes known as HPA backoff).
At the satellite receiver, the received PSK modulated carrier combined with added thermal noise and adjacent channel interference is filtered, amplified, filtered again and transmitted to the receiving earth station.
At the earth station receiver, output thermal noise density N.sub.d is contributed. The filter F4 reduces the total level of noise and interference power before demodulation. The bit error rate, a figure of merit for the entire transmission path, is determined by the levels of thermal noise power, interference power and inter-symbol interference appearing at the demodulator input.
In order to maintain the bit error rate of the transmission channel at acceptable levels, account must be taken of power reduction on the up-link due to rain attenuation (A.sub.u) on the reference channel when similar attenuation is not found in the adjacent channels from stations 1 and 3. This circumstance increases the effect of adjacent channel interference, since the signals contributing to that interference are undiminished whereas the reference signal has been reduced by the rain attenuation. Accordingly, it is exceedingly important to minimize adjacent channel interference, and this has typically been accomplished in the prior art by filtering either during the modulation process or immediately thereafter.
One well-known technique for reducing adjacent channel interference is backing off the HPA so that it operates more, or wholly, in a linear region. This, of course, reduces the output power of the reference channel and, in order to maintain constant power levels, may require provision of a larger HPA. In some systems, typically ones employing a very large number of ground stations, cost constraints require operating the HPA in its near saturation or saturation regions, and amplifier backoff is not an available option. In those situations, of course, reduction of adjacent channel interference is of even greater significance.
Control of adjacent channel interference in the prior art has been accomplished by selecting the appropriate filter 13 or 17 to control spectrum spreading. Typical filters which have been employed include Butterworth, Tschebycheff, Elliptic, or raised cosine types. While such filters do, in fact, exert control on the spectrum spreading problem, their effect is countered, in part, when the HPA is operated in near-saturation or saturation. Operation in this non-linear region increases spectrum spreading by reason of the non-linear amplifier characteristic.
It is therefore one object of the present invention to provide a PSK modulator which exemplifies reduced adjacent channel interference as compared to prior art PSK modulators. It is another object of the present invention to provide a PSK modulator which illustrates this reduction in adjacent channel interference even when an associated HPA is operated in near-saturation or saturation regions. It is a further object of the present invention to provide a PSK modulator with improved filtering action in order to control spectrum spreading and thus reduce adjacent channel interference as compared to prior art PSK modulators. It is another object of the present invention to provide improved filtering in a PSK modulator as aforesaid, which shows improved adjacent channel interference even when an associated HPA is operated in near-saturation or saturation regions.